Automatic frequency control system



S. W. SEELEY AUTOMATIC FREQUENCYv CONTROL SYSTEM .Bune 5, 1945.

Filed Sept. 29, 1942 INVENTOR, v

45 LEX my@ ATTORNEY Patented June 5, 1945 AUTOMATIO FREQUENCY CONTROL SYS TEM Stuart W. Seeley, Roslyn, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application september 29, 1942, serial No. 460,041

(o1. 25o-s6) 21 Claims.

This application concerns a detecting system for automatic frequency control potentials in which the stability of the controlled oscillator is made independent of the usual tuned discriminator circuit. The controlled oscillator output and a reference oscillator output of constant frequency are supplied to a mixer and the intermediate frequency resulting is combined with Wave energy from a second generator of substantially xed frequency and their resultant is fed to a frequency modulation detector and an amplitude modulation detector.

If the oscillator to be controlled is not at exactly the same frequency as the reference oscillations, a beat note can be detected by both amplitude and frequency detecting means.

If the output potentials of the amplitude and frequency detecting means are in phase when the controlled oscillator is higher than the reference oscillator, they will be 180 out of phase with respect to each other when the controlled oscillator is lower in frequency than the reference oscillator. Then, if the potentials derived from the two types of detectors were added together and rectified, the result would be a direct current potential whose magnitude would be equal to that which would be developed in the absence of the frequency detecting means when the beat note is substantially zero and would increase when the controlled oscillator changed its frequency in one direction and woulddecrease when the controlled oscillator changed its frequency in the opposite direction.

However, if the potentials from the two types of detectors are added in a differential rectifier, the output direct current potential is zero when the frequency of the controlled oscillator is substantially equal to that of the reference oscillator but rises in a negative orpositve direction `as the controlled oscillator departs, either higher or lower in frequency, from the reference oscillator. The extent of the rise of the direct current potential in either direction is then proportional to the beat note frequency and thus to the frequency departure.

With automatic frequency control systems as disclosed in frequency modulation systems, such as shown in Crosby U. S. Patent No. 2,279,659, dated April 14, 1942, the frequency of the resulting controlled oscillation is dependent upon the tuning of the discriminator so that if there is a change in the magnitudes of the elements of the discriminator, such as might be brought about by temperature or humidity changes, the frequency of the controlled oscillator varies. In order to overcome this, temperature-controlled discriminators and compensating circuits have been resorted to. In the system of the present invention, the need for such expedients, which compensate for temperature, humidity and other effects on the discriminator, is removed since the developed automatic frequency control potentials depend only on the degree of synchronism between the reference oscillator and the controlled oscillator. The reference oscillator is crystal controlled and of constant frequency.

In describing my invention, reference will be made to the attached drawing wherein the single figure illustrates schematically a wave length modulation system including myvnovel means for deriving a controlling potential which is independent of or unaffected by changes in dimensions or electrical characteristics of the discriminator circuits used to derive the potential.

In the drawing, l0 is a source of oscillations of constant frequency, such as a crystal controlled oscillator. I2 is a wave length modulated wave generator controlled by modulating potentials from source I4 and as will be developed later by automatic frequency control potentials from line I6. The generator l2 is the modulated oscillator, These oscillations are brought together in a mixer 20 and a resulting voltage is combined with oscillations of fixed frequency from source 22. The heterodyned oscillations out of 20 are the controlled oscillations referred to herein. The source 22 is the reference source referred to herein.v

The combined oscillations are fed to a tuned circuit 30. In the arrangement shown, voltages from 20 and 22 are fed into a connection including the inductance 3l shunted by the condenser 33 for combination in series. The voltages produced in circuit 30 are fed to the rst grid 36 of an amplitude limiter and amplitude modulation detector tube 38. The limited voltages are supplied from the output of tube 38 to a discriminator circuit 40 which feeds a pair of diode rectiers 42 and 44 having their cathodes, one of which is grounded, connected in series by resistors 48 and 50, each of which is shunted by a radio-frequency bypass condenser, The voltages developed across resistors 48 and 50 are fed by way of resistance and coupling condenser 64 to a beat frequency amplifier tube 66 and from the amplifier tube by `wayof a transformer 'l0 in phase vto the cathode of a first diode rectifier 'I4 and to the anode of a second diode rectifier 18.

The control grid 36 of tube 38 is also coupled by resistance and coupling condenser 84 to the control grid of a beat frequency amplifier 90,

the output electrodes of which are coupled differentially by transformer 92 to the anode of rectifier 'I4 and the cathode of rectifier '18. An intermediate point on the secondary winding of transformer 92 is connected to ground by resistor and the rectifiers direct current circuit is completed through the secondary winding of transformer 10, which is also grounded at one end. As will be described hereinafter, potentials characteristic of deviations of the controlled generator frequency are developed across resistance I 00 and used in a well known manner to stabilize the frequency of the generator in I2.

The tube 38 and its connection, as stated above, are an amplitude limiter which operates in a well known manner to remove the amplitude variations from the wave supplied to 30. This tube also detects the amplitude variations on the said wave (that is, the beat frequency) and the same appear on the grid 36 due to grid rectification and are applied by resistor V80 and condenser 84 to the grid of amplifier tube 90.

The circuits 43, 45, 41 with rectifiers 42 and 44 comprise a wave length deviation detector of the type disclosed in Seeley U. S. Patent No.,2,121,103, dated June 21, 1938. The operation of this system is believed well known in the art.

The modulated generator at I2 `with a source of modulation at I4 may be of the type disclosed in Crosby U. S. Patent No. 2,279,659, dated April 14, 1942. The automaticfrequency control disclosed here is to replace the automatic frequency vcontrol of the patent.

In my system in 38 I detect the amplitude variations and feed the same by resistor 80 to the amplifier 90. For example, if the transmitter modulated source4 I2 has drifted below its normal frequency, it may be assumed that the controlled output of will have increased by a like amount.

This will provide after mixing with the output of 22 a tone detected in the grid of tube 38 by rectification and supplied to tube 90.

I also supply the amplitude limited voltages with the frequency deviations thereon to a discriminator 40 wherein the frequency variations are detected to set up voltages of the variation frequency in 48 and 50. That is, the frequency varied carrier resulting from beating two waves, one of which may drift in frequency are converted to amplitude variations in the discriminator and demodulated in the usual manner to detect the variations. The detected currents flow differentially in resistances 48 and 50 so .that at the high poten- ;tial lend of resistance 48 we have a tone potential or varying direct current potential, the amplitude and frequency of which depends on the extent of drift of the output of 20 relative to the output of 22.

The amplified tone from the frequency modulation detector is applied to the cathode of rectifier I4 and anode of rectifier 18. The amplified tone from the amplitude modulation detector is applied differentially to the anode of rectifier 14 and cathode 0f rectifier 18. The arrangement is such that during operation the two potentials combine in phase in one side of the audio demodulator'and oppose on the other side depending on whether the output of 20 is above or below the fixed oscillations from 22. If they add in I4 the potential across I00. will be negative. If the heterodyned controlled frequency out of 20 is on the other side of the reference frequency, they become additive in the tube 'I8 and the potential across |00 will be positive.

When the frequency of the modulated oscillator The tone or audio detectors 14 and 1a tnus'supp ply across resistance |00 a direct current potential which is zero when the frequency out of 20 is the same as the frequency out of 22. This potential rises in a negative or positive direction as the controlled oscillations out of 20 depart, either higher or lower in frequency, from the reference oscillations out of 22. The extent of the rise of the direct current potential in either direction is proportional to the beat note frequency and thus to the frequency of departure of the oscillations out of 20 from the proper frequency. The rise is also proportional to the drift of the oscillations in modulated source I2 and in a direction to act through line I6 to retune the generator in I2 to the proper frequency. The potential on line I6 may vcontrol a reactance tube in the modulator of I2 in a sense to return the generator in I2 back to its normal carrier frequency.

In an varrangement tested, the output of the wave generator 22 was 500 k.c. and of constant frequency lwith an amplitude about four times the mean value necessary to operate the limiter 38, while the carrier out of 20 was of 500 k.c. -I- and and of an amplitude about 1/12 the amplitude of the Wave of constant frequency out of 2-2.

The discriminator used at 40 was about five kilocycles wide.

The output of the audio modulation detector 38 fed into the phase detector including tubes 'I4 and 18 was made twice the amplitude of the output of the frequency modulation detector at 2 k.c. This insured proper operation of the phase detector over most of the discriminator range.

What is claimed is:

l. The method of detecting variations in the frequency' of wave energy which may drift in frequency about a mean value which includes the following steps, generating oscillations of fixed frequency substantially equal in frequency to the mean frequency of said wave energy and of amplitude considerably greater than the amplitude of said wave energy, beating said oscillations and said wave energy to derive resultant energyi subjecting said resultant energy to an intensity demodulation process to derive a beat note, subjecting said resultant energy to a frequency demodulation process to derive a beat note, and combining said beat notes to derive a direct current potential which is zero whenl the wave energy and oscillations are of like frequency, positive and proportional to departures of the wave energy frequency from the frequency of the oscillations in one direction and negative and proportional to departures of the wave energy frequency from the frequency of the oscillations in the other direction.

2. The method of stabilizing the mean frequency of Wave energy the mean frequency of which may drift, which includes the following steps, generating oscillations of substantially fixed asvzsev 3 frequency substantially equal in frequency tothe mean frequency of said wave energy, beating said oscillations against said wave energy to produce resulting energy, subjecting said resulting energy to an intensity demodulation process to derive a beat note, subjecting said resulting energy to a frequency demodulation process to derive a beat note, combining said beat notes to'derive a direct current potentialr which is positive and proportional to departures ofthe mean frequency of the wave energy from the frequency of the oscillations in one direction and negative "and proportional to departures of the meanfrequency of the Wave energy from the frequency of the oscillations in the other direction l5 and controlling the mean frequency of the wave energy by said direct current.

3. In a system for producing potentials characteristic of deviations in the frequency of Wave energy, a source of oscillations of substantially constant frequency, means for combining said oscillations with said wave energy to produce a resultant Wave, means for detecting the am-` plitude variations on said resultant wave, means for detecting the frequency variations on said resultant Wave, and a differential rectifier circuit coupled to said last two means` 4. In a system for producing potentials characteristic of deviations in the frequency of Wave energy, a source of oscillations of substantially constant frequency, means for combiningy said oscillations with said wave energy to produce a resultant Wave, means for detecting the intensity Variations on said resultant Wave, means for detecting the frequency variations on said resultant Wave, and a rectifier circuit coupled to said last two means.

5. In a system for producing potentials characteristic of drift in the mean frequency'of Wave energy, a source of oscillations of substantially 40 phases of which reverse as the frequency of the Wave energy passes through the frequency of the oscillations, and a differential rectifier system coupled to said detecting means. y

6. In a system for producing potentials characteristic of drift in the mean frequency of Wave energy, a 'source of oscillations of substantially constant frequency, means for beating said oscillations with said wave energy to derive resultant wave energy, the frequency of which depends on the drift in the mean frequency of the wave energy, means for detecting the amplitude variations, means for detecting the frequency variations, said detecting means providing beat notes the frequency of which depends on the extent of drift and the relative phases of which reverse as the frequency of the Wave energy passes through the frequency of the oscillations, and a rectifier system coupled to said detecting means.

7. In awave length modulation system, a modulator for producing wave energy and modulating the Wave length thereof by signals, said Wave energy being subject to slow changes in the mean frequency thereof, a heterodyne means for reducing the frequency of said wave energy,

a sourceof oscillations of substantially constant A frequency, a mixing circuit for beating said oscillations with said Wave energy of reduced frequency to derive a beat note the frequency of Which ldepends on the extent o f change of mean frequency of said Wave energy of reduced frequency, an amplitude modulation detector for detecting said beat note, a frequency modulation detector for detecting changes in the frequency of said beat note, a modulation combining and rectifying circuit coupled to said last two detectors and a. frequency stabilizing circuit coupling saidl last namedr circuit to said modulator.

8. The method of detecting variations in the frequency of wave energyv which may drift in frequency about a desired frequency whichrincludes these steps, generating oscillations of sub-4 stantially constant frequency substantially equal in frequency tothe desired frequency of said wave energy, beating said oscillations and said waveenergy to derive resultant energy, subjecting said resultant energy to an intensity demodulation process to derive a beat note, subjecting saidresultant energy to a frequency demodulation process to derive a beat note, and `combining and rectifying said beat notes to derive a direct current potential which is zero when the wave energy and generated oscillations are of like frequency, positive when the Wave energy frequency departs from the frequency of the oscillations in one direction and negative when the Wave energy frequency departs from the frequency of the generated oscillations in the other direction. f

9. The method of stabilizingv the mean frequency of' Wave energy, the said mean frequency of which may drift which includes these steps, generating'y oscillations of substantially constant frequency substantially equal in frequency to the mean frequency of said Wave energy, beating said oscillations against said Wave energy to produce resultant energy, subjecting said resultant 'energy to an intensity demodulation process to derive a beat note, subjecting said resultant energy to a frequency demodulation process to derive a beat note, combining and rectifying'said beat notes to derive a direct current potential which is positive when the mean frequency of the wave energy departs from the frequency of the oscillations in one direction, and negative when the mean frequency of the Wave energy departs from the frequency `of the oscillations in the other direction and controlling the mean frequency of said Wave energy by saiddirect current potential.

10. The method of detecting variations in the frequency of wave energy which may drift in frequency about a mean value which includes these steps, generating oscillations of substantially'constant frequency subtsanti'ally equal in frequency to the mean frequency of said wave energy, beating said oscillations and said wave energy -to derive resultant energy, subjecting said resultant energy to an intensity demodulation process' to derive a Wave of beat frequency, limitingthe amplitude of said resultant energy, subjecting said limited resultantenergy to a frequency demodulation process to derive a wave of beat frequency, and combining and rectifying said beat waves to derive a direct current potential Which is zero when the wave energy and oscillations are of like frequency, positive when the wave energy frequency departs from the frequency of the oscillations in one direction and negative when the wave energy frequency departs from the frequency of the oscillations in the other direction.

l1. The method of stabilizing the mean frequency of wave energy, the said mean frequency of .which may drift which; includes these steps, generating oscillations of substantially constant frequency substantially equal in frequency to the mean frequency of said wave energy, beating said oscillations against said Wave energy to produce resultant energy, subjecting said resultant energy to an intensity demodulation process to derive a beat note, limiting the amplitude of the resultant wave energy and subjecting said amplitude limited energy to a frequency demodulation process to derive a beat note, combining and rectifying said beat notes to derive a direct current potential which is positive when the mean frequency of the wave energy `departs from the frequency of the oscillations in one direction, and negative when the mean frequency of the Wave energy departs from the frequency of the oscilfj lations` in the other direction and controlling the mean frequency of said wave energy by said direct current potential.

12. In a system for producing potentials char-v acteristic of the deviations in the mean frequency of'wave energy,` a source of oscillations of substantially constant frequency, a mixer stage for combining said loscillations with said Wave energy to produce resultant wave energy, a frequency modulation detector coupled tosaid mixer stage for detecting the frequency variations of such resultant wave energy, an amplitude modulation detector circuit coupled to said mixer stage for detecting the amplitude variations of said resultant Wave energy and a differential rectier circuitcoupled to said detectors in such a manner that the output of said differential rec--` tier is zero when the frequency of the oscillations is equal to the mean frequency of the wave energy,` is positive when the frequency of the wave energy is on one side of the frequency of said oscillations and is negative when the frequency of the wave energy is on the other side of the frequency of said oscillations.

13. In a system for producing potentials characteristic of deviations in the mean frequency of wave energy, a source of oscillations of substantially constant frequency, a mixer stage for combining said oscillations with said wave energy t produce resultant Wave energy, an. amplitude modulation detecting circuit coupled to said mixer stage for detecting the amplitude variations of said resultant wave energy, an amplitude limiter circuit and frequency modulation converting and detecting apparatus coupled to said mixer stage for detecting the frequency variations of said resultant wave energy, and a differential rectifier circuit coupled to said detectors in such a manner that the output of said differential rectifier' is zero when the frequency of the oscillations is equal to the mean frequency ofthe wave energy, is positive When the frequency of the Wave energy is on one side of the frequency of said oscillations and is negative when the frequency of the wave energy is on the other side of the frequency of said oscillations.

14. In a system for producing potentials characteristic of variations in the mean frequency of wave energy, a source of oscillations of substantially constant frequency, a wave converter Wave amplitude limiter andwave amplitude detector for beating said oscillations with said wave energy to derive yresultant wave energy, and detecting amplitude variations on resultant wave energy and limiting the amplitude of resultant wave energy, a wave amplifier excited by the detected amplitude variations, a frequency discrimwww inator and detector excited by the resultant wave energy of limited amplitude, a wave ampliner coupled to the last named detector, said defec-4 tors providing beat notes the relative phases of which `reverse as the frequency of the Wave energy passes through the frequency of the oscillations, and a differential rectifier system coupled to said Waveampliers in such a. manner that the output of said differential rectifier system is zero, if the frequency of thewave energy is equal to the frequency of the oscillations, and is negative when the frequency of the wave energy departs fom the frequency of the oscillations in one direction and positive when the frequency of ther Wave energy departs from the frequency of the oscillations in the other direction.

15. In a system for stabilizing the mean frequency of wave energy the mean frequency of which may vary, a source of oscillations of substantially constant frequency, a wave converter, Wave amplitude limiter and converted wave amplitude detector for beating said oscillations with said wave energy to derive resultant Wave energy, and detecting amplitude variations of resultant Wave energy and limiting the amplitude of resultant wave energy, a frequency discriminator and detector excited by the limited output, said detectors providing beat notes the relative phases of which reverse as the frequency of the wave energy passes through the frequency of the oscillations, a differential rectifier system coupled to said detectors in lsuch a manner that the output of said differential rectifier system is zero, if the frequency of the wave energy is equal to the frej quency of the oscillations, and is negative when the frequency of the Wave energy departs from the frequency of the oscillations in one direction andvpositive when the frequency of the wave energy departs from the frequency of the oscillations n the other direction, and connections for controlling the frequency of said wave energy in accordance with the output of the rectifier system.

16. `In a systemfor producing potentials characteristic of variations in the mean frequency of the wave energy, the mean or average frequency of which may drift, a sourceof oscillations of substantially constant frequency, a mixer stage excited by oscillations from said source and by said wave energyfor beating one against the other to derive resultant wave energy the frequency and amplitude of which varies in accordance with variations in the mean frequency of the wave energy, an amplitude modulation detector circuit coupled to said mixer stage for detecting the said amplitude variations, a frequency modulation detector coupled to said mixer stage for detecting the frequency variations, said detectors providing beat notes the relative phases of which reverse as the frequency of the Wave energy passes through the frequency of the oscillations, a pair of rectifier devices having corresponding electrodes, a first impedance, a load, a second impedance, connections coupling said 'rectifiers in opposed relation in a series circuit including said first impedance and said load, connections including said second impedance in a bridge circuit with said rectifiers, with said load impedance in the diagonal of the bridge, and connections for applying the output of one detector on one-of said impedances and the output ofthe other .detector on the other of said impedances.

1'7. The method which includes combining a wave which may vary in frequency with a wave of substantially constant frequencm; deriving from the combined waves two beats, one 'of winch results from an amplitude demodulation and the other of which results from a frequency demodulation of a portion of the combined waves and detecting the changes in phase of the beats as said Waves pass through synchronism.

18. In a system for producing potentials characteristic of deviations in the frequency of wave energy, a source of oscillations of substantially constant frequency, a combining circuit excited by said wave energy and coupled to said source for beating said oscillations with said wave energy to modulate the amplitude and length of the oscillations in accordance with the deviations in the wave energy, a demodulator for detecting the amplitude modulations on said oscillations, a demodulator for simultaneously detecting the wave length modulations on said oscillations, and a modulation combining and rectifying circuit coupled to said demodulators.

19. In a wave length modulation system, a controlled wave generator for producing Wave energy and mo'dulating the wave length thereof by signals, said Wave energy being subject to slow changes in the mean frequency thereof, a source of oscillations of substantially constant frequency,

a combining circuit coupled to said Wave gen- 20. In a. system of the class described, a source of wave energy the frequency of which may deviate about a selected frequency, a source of oscillations of substantially constant frequency substantially equal to said selected frequency, means for combining said oscillations and wave energy to modulate the phase and amplitude of the oscillations in accordance With said frequency deviations, means for detecting both of said modulations to derive two beat notes one of which reverses in phase when the Wave energy frequency passes through the frequency of said osci1 lations, and means for deriving a potential equal to the summation of the beat notes.

21. In a system of the class described, a source of wave energy the frequency of which may deviate aboutl a selected frequency, a source o f oscillations of substantially constant frequency substantially equal to said selected frequency, means for combining said oscillations and said Wave energy to modulate the length and amplitude of said oscillations, means for detecting said modulated oscillations to obtain a beat note characteristic of said amplitude modulations and a beat note characteristic of said phase modulations, there being a relative reversal of phase of said beat notes when the Wave energy frequency passes through the frequency of the oscillation, means for combining said beat notes in phase or in phase opposition depending on the relative frequencies of said wave energy and oscillations, and means for rectifying said combined beat notes to obtain a potential characteristic of the said variations of said wave energy.

STUART W. SEELEY. 

